Short Report

Is this a 316 failure?

Hmm! it appears we have A4 (316) SS failing on a sulphate crag? It’s time to measure stuff.

Many thanks to Simon Alden for the sample and photographs.

This story comes from the sea cliffs of Malta. Whilst I don’t have significant geochemical sampling for Malta, I have every reason to believe the Mediterranean location will encourage the presence of sulphate. See this earlier post for a detailed explanation. To date, I have one positive identification of a 304 bolt from Malta that has failed under the attack of sulphate reducing bacteria (SRB), so I guess another is possible…. but 316?

Some ten years ago Simon Alden installed a couple of 316 SS twist bolts from Bolt Products using Hilti RE500. This route on Gozo is rarely climbed, and it was only during a recent visit that Simon was surprised to see both anchors were clearly cracked.

Location of bolts at the top of a picturesque Gozo sea cliff

Simon cut the bolts off and sent me the left hand one. My first action on receiving it was to check attraction to my strong, rare-earth magnet. This is an important test, and I did it even before taking the sample out of the bag. It seemed strongly attracted to the magnet.

Whoops! If that is 316 then it must have been formed by elves in Santa’s Arctic workshop. At room temperature this can’t happen. Another possibility it is out of spec. for 316.

We can get a lot more exacting than “seems strongly attracted to a magnet”. To this end, I have developed a simple technique where I measure the force required to pull a tiny rare-earth magnet away from a contact point on the surface of the bolt. By various empirical means, all imperfect, I have arrived at the approximate relationship between that force and the percentage strain-induced α-martensite at the contact point.

According to my rough calibration, we are seeing 58% martensite here, and, based on the rate of diffusion of atomic hydrogen through austenitic steel containing different fractions of martensite, I wouldn’t expect that bolt to survive in a sulphate environment. This prediction accords with the level of damage we see.

The effect of martensite fraction on the time taken for atomic hydrogen to diffuse 2mm. See my post on the subject … it’s complicated!

Having decided that this bolt couldn’t be 316, a bit of analytical chemistry was called for. This is what I found. Methodology is here.

elementcomposition (%)error (%)
Nickel8.4+/- 0.2
Chromium18.1+/- 0.1
Molybdenum< 0.1
Elemental analysis excludes 316 and points to 304.

My conclusion is that we are looking at a 304 failure, not a 316 failure.

Evidence for SRB attack on 304 anchors located within the Mediterranean, and along the Portuguese coast, is gradually accumulating. Failure modes that were originally attributed to SCC (stress corrosion cracking) are beginning to be revealed as SSC (sulphide stress cracking).

What evidence can we find for that mechanism?

In cross-section the stress cracking is obvious.

Cross-section of fracture region showing multiple stress fractures. The bolt is 6mm in diameter.

The real test for an SSC mechanism is to demonstrate the presence of sulphide.

This was easily done using the Iodine-Azide spot test.

Positive iodine-azide test for metal sulphide for deposit adjacent to the crack.

A further tell-tale sign of SRB releasing sulphide is the formation of the mineral greigite, Fe3S4. The deposition of this particular iron sulphide is likely to be favoured by pH and oxygen level at some point between the inside of the bolt and the free atmosphere. See this post for a discussion of the chemistry. It is quite distinctive with iridescent, octahedral/cubic crystal facets. The picture below shows the greigite deposit I found.

Crystaline deposit of the iron sulphide mineral greigite…. a sure indicator that SRB have been active.

If we take a close look at a polished section of the cracked region, we find the metal to be fragmented and snapped on the microscale. The grain size, not observable in the photos, is of the order of 10um to 20um, whereas we are seeing fractures occurring on a geometric grid with a spacing closer to 1um. This is characteristic of hydrogen embrittlement where, according to the HELP hypothesis, micro-voiding is initiated at the intersection of the slip planes of the lattice. I have a 101 introduction to the subject in this post.

Polished section through zone of fracture. Note the geometrically ordered multiple fractures in the micron size range (red arrow). This once ductile metal is now brittle. Oil-immersion optical microscopy with electrolytic etching in 50% nitric acid.

So, we observe a ductile material rendered totally brittle at the microscale. It is hard to imagine that such a tough alloy as 304 would powder to dust at the points of maximum stress, yet that is what the photo below reveals. The powdered metal at the fracture has been caught up in the epoxy encapsulant that I use to embed the specimen under vacuum.

Polished section through zone of fracture. This region is made up of finely ‘powdered’ metal embedded in the epoxy resin encapsulant. Note the scale. This once ductile metal is now brittle. Oil-immersion optical microscopy with electrolytic etching in 50% nitric acid.

So, we see once again the attack of SRB on 304 rock anchors. The observed stress fractures are SSC not SCC and beneath the surface the hydrogen embrittlement that enables this flavor of stress cracking is obvious.

The challenge still stands: Bring me a sample of a 316 anchor that has failed through SRB attack.

3 replies on “Is this a 316 failure?”

Hi Dave. Thanks for another interesting post. I cannot avoid but think about the bolts from Laguna. At a layman’s eye they look very similar, except that there they were more yellowish all around instead of shiny. I’m not sure there were visible cracks as well.
Best wishes

I owe you an apology for being so slow with that sample. I’m just in the process of preparing a section for microscopy.

It doesn’t look like SRB attack and doesn’t test positive for sulphide. I was thinking it was more likely a metallurgical problem, but it is not hard for 304, so over-hardening looks unlikely.

Let’s see what I can see in the microstructure. I suspect we have something quite different here.

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